Catalytically active particle filter with a high degree of filtration efficiency

ABSTRACT

The invention relates to a wall-flow filter as a particle filter with catalytically active coatings in the channels which are closed in a gas-tight manner at the opposing closed ends of the channels A at the first end, wherein the inlet region of the filter is additionally supplied with a dry powder-gas aerosol which contains metal compounds with a high melting point (such as the metal oxides Al203, Si02, Fe02, Ti02, Zn02, etc. for example) and which is to simultaneously improve the catalytic activity and the degree of filtration efficiency with respect to the exhaust gas back-pressure.

The invention relates to a wall-flow filter, a method for its productionand the use of the filter in the reduction of harmful exhaust gases andfine particles of an internal combustion engine.

Diesel particulate filters or gasoline particulate filters with andwithout an additional catalytically active coating are suitableaggregates for removing particle emissions and reducing harmfulsubstances in exhaust gases. These are wall-flow honeycomb bodies, whichare referred to as catalyst supports, carriers or substrate monoliths.In order to meet the legal standards, it is desirable for current andfuture applications for the exhaust gas aftertreatment of internalcombustion engines to combine particulate filters with othercatalytically active functionalities not only for reasons of cost butalso for installation space reasons. The catalytically active coatingcan be located on the surface or in the walls of the channels formingthis surface. The catalytically active coating is often applied to thecatalyst support in the form of a suspension in a so-called coatingoperation. Many such processes in this respect were published in thepast by automotive exhaust-gas catalytic converter manufacturers(EP1064094B1, EP2521618B1, WO10015573A2, EP1136462B1, U.S. Pat. No.6,478,874B1, U.S. Pat. No. 4,609,563A, WO9947260A1, JP5378659B2,EP2415522A1, JP2014205108A2). The use of a particulate filter, whethercatalytically coated or not, leads to a noticeable increase in theexhaust-gas back pressure in comparison with a flow-through support ofthe same dimensions and thus to a reduction in the torque of the engineor possibly to increased fuel consumption. In order to not increase theexhaust-gas back pressure even further, the amounts of oxidic supportmaterials for the catalytically active noble metals of the catalyticconverter or oxidic catalyst materials are generally applied in smallerquantities in the case of a filter than in the case of a flow-throughsupport. As a result, the catalytic effectiveness of a catalyticallycoated particulate filter is frequently inferior to that of aflow-through monolith of the same dimensions.

There have already been some efforts to provide particulate filters thathave good catalytic activity due to an active coating and yet have thepreferably low exhaust-gas back pressure. With regard to a lowexhaust-gas back pressure, it has proven to be advantageous if thecatalytically active coating is not present as a layer on the channelwalls of a porous wall-flow filter; rather, the channel walls of thefilter are instead interspersed with the catalytically active material(WO2005016497A1, JPH01-151706, EP1789190B1). For this purpose, theparticle size of the catalytic coating is selected such that theparticles penetrate into the pores of the wall-flow filters and can befixed there by calcination. A disadvantage of catalytically activefilters having an in-wall coating is that the amount of catalyticallyactive substance is limited by the absorption capacity of the porouswall.

It has been found that, by applying the catalytically active substancesto the surfaces of the channel walls of a wall-flow honeycomb body, anincrease in the conversion of the harmful substances in the exhaust gascan be achieved. Combinations of on-wall coating and in-wall coatingwith catalytically active material are also possible, as a result ofwhich the catalytic performance can be further increased withoutsubstantially increasing the back pressure.

In addition to the catalytic effectiveness, a further functionality ofthe filter that can be improved by a coating is its filtrationefficiency, i.e., the filtering effect itself. WO 2011151711A1 describesa method by means of which a dry aerosol is applied to a non-coated orcatalytically coated filter that carries the catalytic active materialin the channel walls (in-wall coating with a washcoat). The aerosol isprovided by the distribution of a powdery metal oxide with a highmelting point and guided through the inlet side of a wall-flow filter bymeans of a gas stream. In this case, the individual particles having aparticle size of 0.2 μm to 5 μm agglomerate to form a bridged network ofparticles and are deposited as a layer on the surface of the individualinlet channels passing through the wall-flow filter. The typical powderloading of a filter is between 5 g and 50 g per liter of filter volume.It is expressly pointed out that it is not desirable to obtain a coatinginside the pores of the wall-flow filter with the metal oxide.

A further method for increasing the filtration efficiency ofcatalytically inactive filters is described in WO2012030534A1. In thiscase, a filtration layer (“discriminating layer”) is created on thewalls of the flow channels of the inlet side by the deposition ofceramic particles via a particle aerosol. The layers consist of oxidesof zirconium, aluminum or silicon, preferably in fiber form ranging from1 nm to 5 μm, and have a layer thickness greater than 10 μm, typically25 μm to 75 μm. After the coating process, the applied powder particlesare calcined in a thermal process.

A further method in which a membrane (“trapping layer”) is produced onthe surfaces of the inlet channels of filters in order to increase thefiltration efficiency of catalytically inactive wall-flow filters isdescribed in patent specification U.S. Pat. No. 8,277,880B2. Thefiltration membrane on the surfaces of the inlet channels is produced bysucking a gas stream loaded with ceramic particles (for example, siliconcarbide, cordierite) through. After application of the filter layer, thehoneycomb body is fired at temperatures greater than 1000° C. in orderto increase the adhesive strength of the powder layer on the channelwalls. EP2502661A2 and EP2502662B1 mention further on-wall coatings bypowder application.

A coating inside the pores of a wall-flow filter unit by spraying dryparticles is described in U.S. Pat. No. 8,388,721 B2. In this case,however, the powder should penetrate deeply into the pores. 20% to 60%of the surface of the wall should remain accessible to soot particles,thus open. Depending on the flow velocity of the powder/gas mixture, amore or less steep powder gradient between the inlet and outlet sidescan be set. The pores of the channel walls of the filter coated withpowder in the pores according to U.S. Pat. No. 8,388,721B2 cansubsequently be coated with a catalytically active component. Here aswell, the catalytically active material is located in the channel wallsof the filter.

The introduction of the powder into the pores, for example by means ofan aerosol generator, is also described in EP2727640A1. Here, anon-catalytically coated wall-flow filter is coated using a gas streamcontaining, for example, aluminum oxide particles in such a way that thecomplete particles, which have a particle size of 0.1 μm to 5 μm, aredeposited as a porous filling in the pores of the wall-flow filter. Theparticles themselves can realize a further functionality of the filterin addition to the filtering effect. For example, these particles aredeposited in the pores of the filter in an amount greater than 80 g/lbased on the filter volume. They fill in 10% to 50% of the volume of thefilled pores in the channel walls. This filter, both loaded with sootand without soot, has an improved filtration efficiency compared to theuntreated filter together with a low exhaust-gas back pressure of thesoot-loaded filter.

In WO2018115900A1, wall-flow filters are coated with an optionally drysynthetic ash in such a way that a continuous membrane layer is formedon the walls of the optionally catalytically coated wall-flow filter.

All patents listed in this prior art have the aim of increasing thefiltration efficiency of a filter by coating the filter with a powder.The filters optimized in this way with regard to filtration efficiencycan also carry a catalytically active coating in the porous channelwalls before the powder coating. However, there are no indications inany of the examples to simultaneously optimize the catalytic effect of afilter and increase filtration efficiency. Therefore, there continues tobe a need for particulate filters with which both catalytic activity andfiltration efficiency are optimized with respect to exhaust-gas backpressure.

The object of the present invention is to specify a correspondingparticulate filter with which a sufficient filtration efficiency iscoupled with the lowest possible increase in the exhaust-gas backpressure and a high catalytic activity.

These and other objects that are obvious from the prior art are achievedby the specification of a particulate filter according to claims 1 to13. Claims 14 to 16 are aimed at the production of a particulate filteraccording to the invention. Claim 17 aims at using the particulatefilter for the exhaust-gas aftertreatment of internal combustionengines.

The present invention relates to a wall-flow filter for removingparticles from the exhaust gas of internal combustion engines, whereinsuch wall-flow filter has a length L and at least one catalyticallyactive coating Y and/or Z and channels E and A, which extend in parallelbetween a first and a second end of the wall-flow filter and areseparated by porous walls, and form the surfaces O_(E) or O_(A), andwherein the channels E are closed at the second end and the channels Aare closed at the first end. The object posed is very surprisinglyachieved in that in such a wall-flow filter, the coating Y is located inthe channels E on the walls of the surfaces O_(E), wherein the coatingon the walls of the surface O_(E) extends from the first end of thewall-flow filter to a length of less than the length L, and the coatingZ is located in the channels A on the walls of the surfaces O_(A),wherein the coating on the walls of the surface O_(A) extends from thesecond end of the wall-flow filter to a length of less than the lengthL, and wherein the inlet region E of the filter has additionally beenimpinged with at least one dry powder-gas aerosol. Impinging a wall-flowfilter that is conventionally zone-coated using wet techniques (asdescribed in U.S. Pat. No. 8,794,178, for example) and in which thecatalytically active coating is located on the surface of the channelwalls and that is coated after drying and/or calcination with a drypowder-gas aerosol, results in a wall-flow filter with extremely goodfiltration efficiency and only slightly increased exhaust-gas backpressure and simultaneously excellent catalytic activity.

The filters described herein, which are catalytically coated and thenimpinged with powder, differ from those that are produced in the exhaustsystem of a vehicle by ash deposition during operation. According to theinvention, the catalytically active filters are deliberatelypowder-sprayed with a specific, dry powder. As a result, the balancebetween filtration efficiency and exhaust-gas back pressure can beadjusted selectively right from the start. Wall-flow filters with whichundefined ash deposits have, for example, resulted from fuel combustionin the cylinder during driving operation or by means of a burner aretherefore not included in the present invention.

All ceramic materials customary in the prior art can be used aswall-flow monoliths or wall-flow filters. Porous wall-flow filtersubstrates made of cordierite, silicon carbide or aluminum titanate arepreferably used. These wall-flow filter substrates have inflow andoutflow channels, wherein the respective downstream ends of the inflowchannels and the upstream ends of the outflow channels are offsetagainst each other and closed off with gas-tight “plugs.” In this case,the exhaust gas that is to be purified and that flows through the filtersubstrate is forced to pass through the porous wall between the inflowchannel and outflow channel, which brings about a particulate filteringeffect. The filtration property for particulates can be designed bymeans of porosity, pore/radii distribution, and thickness of the wall.The porosity of the uncoated wall-flow filters is typically more than40%, generally from 40% to 75%, particularly from 50% to 70% [measuredaccording to DIN 66133, latest version on the date of application]. Theaverage pore size of the uncoated filters is at least 7 μm, for examplefrom 7 μm to 34 μm, preferably more than 10 μm, in particular morepreferably from 10 μm to 25 μm or very preferably from 15 μm to 20 μm[measured according to DIN 66134, latest version on the date ofapplication]. The completed (catalytically coated) filters with a poresize of typically 10 μm to 20 μm and a porosity of 50% to 65% areparticularly preferred.

The catalytic coatings Y and Z of the wall-flow filter to be impingedwith the powder do not extend over the entire length of the wall-flowfilter. The coatings Y and/or Z (if present) are preferably locatedstarting from the respective end of the wall-flow filter to a length ofup to 90% of the length L. The minimum length of the coatings Y and Z,if present, is at least 1.25 cm, preferably at least 2.0 cm and verypreferably at least 2.5 cm, as calculated from the respective end of thefilter. However, the coatings Y and Z, if present, are each located onthe surfaces O_(E) and/or O_(A). These so-called on-wall coatings arepreferably from 10 to 90% of the length L of the wall-flow filter,preferably 30 to 80%, and particularly preferably 60 to 80%, ascalculated from the respective end of the wall-flow filter. The term“on-wall coating” means that these coatings rise above the surfacesO_(E) and O_(A) into the channels E and A of the wall-flow filter,respectively, and consequently reduce the channel cross section. Thesuperficial pores of the surfaces O_(E) and O_(A) are only secondarilyfilled with the catalytically active material. More than 80%, preferablymore than 90%, of the catalytically active material is not located inthe porous wall of the channels E and A.

In a further preferred embodiment, the coating Y and/or Z (if present)has a thickness gradient over the length L such that the least thicknessof the coating Y and/or Z prevails at the respective ends of thewall-flow filter. Consequently, the thickness increases along the lengthL of the wall-flow filter (see FIG. 3 ). In this case, such coatings maypreferably have more than 2 times, more preferably up to more than 3times, the thickness at one coating end than at the other coating end.In this case, the thickness is the height at which the coating Y and/orZ rises above the surface O_(E) or O_(A). The thickness gradient of thecoating on the channel walls also makes it possible for the filtrationefficiency to be adjusted over the entire length L of the filter. Theresult is a more uniform deposition of the soot over the entire filterwall and thus an improved exhaust-gas back pressure increase andpossibly a better burn-off of the soot.

In addition to the catalytically active coatings Y and Z, the wall-flowfilter according to the invention can have a further coating X locatedin its walls and extending from the first or second end of the wall-flowfilter to a length of up to 100% of the length L. The minimum of suchin-wall coating is at least 1.25 cm, preferably at least 2.0 cm and verypreferably at least 2.5 cm, as calculated from the respective end of thefilter. The coating X preferably extends over up to 80% of the length L.

Accordingly, the wall-flow filter, which is impinged upon according tothe invention with the powder-gas aerosol, already contains withinitself a catalytic activity (referred to herein as X, Y and Z). Here,catalytic activity is understood to mean the ability to convert harmfulconstituents of the exhaust gas from internal combustion engines intoless harmful ones. The exhaust gas constituents NOx, CO and HC or theparticulate burn-off should be mentioned here in particular. Suchcatalytic activity is provided according to the requirements of theperson skilled in the art by a coating of the wall-flow filter in itswalls and/or on its walls with a catalytically active material. The term“coating” is accordingly to be understood to mean the application ofcatalytically active materials to a wall-flow filter. The coatingassumes the actual catalytic function. In the present case, the coatingis carried out by applying a correspondingly low-viscosity aqueoussuspension of the catalytically active components, also referred to as awashcoat, into or onto the wall of the wall-flow filter, e.g., inaccordance with EP1789190B1. After application of the suspension, thewall-flow filter is dried and, if applicable, calcined at an increasedtemperature. The catalytically coated filter preferably has a loading of20 g/l to 200 g/l, preferably 30 g/l to 150 g/l. The most suitableamount of loading of a filter coated in the wall depends on its celldensity, its wall thickness, and the porosity.

In principle, all coatings known to the person skilled in the art forthe automotive exhaust-gas field along with combinations thereof aresuitable for the present invention. The catalytically active coating ofthe filter can preferably be selected from the group consisting ofthree-way catalyst, SCR catalyst, nitrogen oxide storage catalyst,oxidation catalyst, soot-burn-off coating. With regard to the individualcatalytic functions coming into consideration and their explanation,reference is made to the statements in WO2011151711A1.

Particular preference is given to coatings (for X, Y and/or Z) that havea three-way functionality and are active in particular at operatingtemperatures of 250 to 1100° C. They usually contain one or more noblemetals, which are fixed on one or more carrier materials, along with oneor more oxygen storage components. Platinum, palladium and rhodium areparticularly suitable as precious metals, wherein palladium, rhodium orpalladium and rhodium are preferred and palladium and rhodium areparticularly preferred. Based on the particulate filter according to theinvention, the proportion of rhodium in the entire noble metal contentis in particular greater than or equal to 10% by weight. The noblemetals are usually used in quantities of 0.15 to 5 g/l based on thevolume of the wall-flow filter.

As carrier materials for the precious metals, all materials familiar tothe person skilled in the art for this purpose can be considered. Suchmaterials are in particular metal oxides with a BET surface area of 30to 250 m²/g, preferably 100 to 200 m²/g (determined according to DIN66132—latest version as of filing date). Particularly suitable carriermaterials for the precious metals are selected from the seriesconsisting of aluminum oxide, doped aluminum oxide, silicon oxide,titanium dioxide and mixed oxides of one or more of these. Dopedaluminum oxides are, for example, aluminum oxides doped with lanthanumoxide, zirconium oxide and/or titanium oxide. Lanthanum-stabilizedaluminum oxide is advantageously used, wherein lanthanum is used inquantities of 1 to 10% by weight, preferably 3 to 6% by weight, in eachcase calculated as La₂O₃ and based on the weight of the stabilizedaluminum oxide. Another suitable carrier material islanthanum-stabilized aluminum oxide the surface of which is coated withlanthanum oxide, with barium oxide or with strontium oxide.

Cerium/zirconium/rare earth metal mixed oxides are particularly suitableas the oxygen storage component. The term, “cerium/zirconium/rare-earthmetal mixed oxide,” within the meaning of the present invention excludesphysical mixtures of cerium oxide, zirconium oxide, and rare earthoxide. Rather, “cerium/zirconium/rare earth metal mixed oxides” arecharacterized by a largely homogeneous, three-dimensional crystalstructure that is ideally free of phases of pure cerium oxide, zirconiumoxide or rare earth oxide (fixed solution). Depending on themanufacturing process, however, not completely homogeneous products mayarise which can generally be used without any disadvantage. In all otherrespects, the term “rare earth metal” or “rare earth metal oxide” withinthe meaning of the present invention does not include cerium or ceriumoxide. Lanthanum oxide, yttrium oxide, praseodymium oxide, neodymiumoxide and/or samarium oxide can, for example, be considered as rareearth metal oxides in the cerium/zirconium/rare earth metal mixedoxides. Lanthanum oxide, yttrium oxide and/or praseodymium oxide arepreferred. Lanthanum oxide and/or yttrium oxide are particularlypreferred, and lanthanum oxide and yttrium oxide, yttrium oxide andpraseodymium oxide, and lanthanum oxide and praseodymium oxide are moreparticularly preferred. In embodiments of the present invention, theoxygen storage components are free of neodymium oxide. In accordancewith the invention, the cerium oxide to zirconium oxide mass ratio inthe cerium/zirconium/rare earth metal mixed oxides can vary within widelimits. It is, for example, 0.1 to 1.5, preferably 0.2 to 1 or 0.3 to0.5.

If the cerium/zirconium/rare earth metal mixed oxides contain yttriumoxide as a rare earth metal, the proportion thereof is in particular 5to 15% by weight based on the oxygen storage material. If thecerium/zirconium/rare earth metal mixed oxides contain praseodymiumoxide as a rare earth metal, the proportion thereof is in particular 2to 10% by weight based on the oxygen storage material. If thecerium/zirconium/rare earth metal mixed oxides contain lanthanum oxideand yttrium oxide as a rare earth metal, the mass ratio thereof is inparticular 0.1 to 1. If the cerium/zirconium/rare earth metal mixedoxides contain lanthanum oxide and praseodymium oxide as a rare earthmetal, the mass ratio thereof is in particular 0.1 to 1. The coatings Y,Y′ and Z usually contain oxygen storage components in quantities of 15to 120 g/l, based on the volume of the wall-flow filter. The mass ratioof carrier materials and oxygen storage components in the coatings X, Yand Z (if present) is usually 0.3 to 1.5, for example 0.4 to 1.3.

In embodiments of the present invention, the coatings X and Y along withZ (if present) comprise lanthanum-stabilized aluminum oxide, rhodium,palladium or palladium and rhodium, and an oxygen storage componentcomprising zirconium oxide, cerium oxide, yttrium oxide and lanthanumoxide. In other embodiments of the present invention, the coatings X andY along with Z (if present) comprise lanthanum-stabilized aluminumoxide, rhodium, palladium or palladium and rhodium, and an oxygenstorage component comprising zirconium oxide, cerium oxide, praseodymiumoxide and lanthanum oxide.

In embodiments of the present invention, the sum of the lengths ofcoating Y and coating Z, if present, is preferably 110 to 160% of thelength L. In another preferred embodiment, the sum of the lengths ofcoating X and coating Z, if present, is preferably 90 to 50% of thelength L. In embodiments of the present invention, the coatings X, Y andZ contain no zeolite and no molecular sieve.

The composition of the particular coating and the type of coating(in-wall or on-wall) along with the amount of catalytic coatings appliedcan be varied and combined with regard to the desired catalyticactivity. The above-described variation of the catalytically activecoating creates different wall permeabilities in the filter. Inaddition, as a result of production, the catalytic layers on the filterwalls sometimes have inhomogeneities, small cracks, different layerthicknesses and partly also uncoated defects, and although they have ahigh catalytic activity and an acceptable back pressure, they have arelatively low fresh filtration efficiency. The regions which have onlyan in-wall coating X or no catalytic coating may also have a lowerfiltration efficiency. All of this has the result that the channel wallsare gas-permeable to different extents and can have regions withdifferent gas permeability.

Impinging the wall-flow filter considered here with the dry powder-gasaerosol thus leads to the powder particles, following the flow of thegas, depositing on the surface of the input side of the filter andpossibly in the pores of the filter. In the process, the aforementioneddifferent wall permeability of the filter (e.g., due to inhomogeneitiesof the filter wall itself or different coating zones) leads to aselective deposition of the powder on the filter wall or in the pores ofthe wall where the flow is the greatest. This effect also results in,for example, cracks or pores in the washcoat layer being filled up bythe porous powder due to coating defects, such that the soot particlesin the exhaust gas are later increasingly retained as the exhaust gaspasses through the filter. A better filtration efficiency isconsequently the result.

According to the invention, the zone-coated, dry exhaust-gas filter isthus coated in the exhaust-gas flow direction with a powder in such away that the cell wall regions with the highest flow-through are coatedby loose, intrinsically porous powder accumulations in the pores and/oralso on the wall in order to obtain a desired increased filtrationefficiency. In the process, the formation of the intrinsically porouspowder accumulations surprisingly leads to a relatively low increase inback pressure. In a preferred embodiment, the wall-flow filter isimpinged with a powder-gas aerosol such that during impinging, thepowder deposits on the channel walls of the input side and builds up acohesive layer there. In a further quite preferred embodiment, thewall-flow filter is impinged with a powder-gas aerosol such that duringimpinging, the powder precipitates in the pores of the filter walls andfills them as far as the input surface and thereby does not form acohesive layer on the walls of the filter.

The amount of powder used depends on the type of powder, the type ofcoating (see just above) and optionally the volume of the availablepores and can be determined by the person skilled in the art inpreliminary experiments under the given boundary conditions (anexhaust-gas back pressure that is not too high). As a rule, the loadingof the filter with the powder is no more than 50 g/l based on the filtervolume. The value is preferably not more than 20 g/l, very particularlypreferably not more than 10 g/l. A lower limit is naturally formed bythe desired increase in filtration efficiency. The exact amount ofpowder needed to increase filtration efficiency is determined dependingon the length and type of the catalytic coating. If only one powdercoating is desired in the surface pores of the filter, an amount ofpowder of up to 10 g/l, preferably up to 5 g/l, is advantageouslysufficient under the stated conditions.

Powders which are preferably used in the present invention for producingthe aerosol are sufficiently familiar to the person skilled in the art.These are generally high-melting metal compounds, which are commonlyused as support materials for catalysts in the automotive exhaust-gasfield. Corresponding metal oxide, metal sulfate, metal phosphate, metalcarbonate or metal hydroxide powders or their mixtures are preferablyused. Possible metals for the metal compounds are in particular thoseselected from the group of alkali metals, alkaline earth metals or earthmetals, or transition metals. Such metals selected from the group ofcalcium, magnesium, strontium, barium, aluminum, silicon, titanium,zirconium, cerium are preferably used. As stated, these metals canpreferably be used as oxides. Very particular preference is given to theuse of cerium oxide, titanium dioxide, zirconium dioxide, silicondioxide, aluminum oxide, iron oxide, zinc oxide or mixtures or mixedoxides thereof. Very particular preference is given to the use of anaerosol which is a mixture of air and one of these metal oxide powders.Here, the term “mixed oxide” (solid solutions of a metal oxide in atleast one other) is also understood to mean the use of zeolites andzeotypes. In the context of the invention, zeolites and zeotypes aredefined as in WO2015049110A1.

In addition to the usually chemically precipitated metal oxides,so-called pyrogenic metal oxide powders can also be used. In general,pyrogenically produced metal oxide powders are understood to be thoseobtained by flame hydrolysis or flame oxidation from a metal oxideprecursor in an oxyhydrogen gas flame(https://de.wikipedia.org/w/index.php?title=Pyrogenes_Siliciumdioxid&oldid=182147815;Pater Albers et al., Chemie in unserer Zeit [Chemistry in our Time],2016, 50, 162-171; Hans Ferkel et al., MTZ—Motortechnische Zeitschrift[Motor Engineering Journal], 2010, 71, 128-133). Such powders haveproperties as described for flame-synthesized particulate products inthe following references: Gutsch et al. (2002) KONA (no. 20); Li S. etal. (2016) Progress in Energy and Combustion Science (55); Ulrich G.(1971) Combustion Science and Technology (vol. 4). Pyrogenic metaloxides are generally characterized by a high specific surface area and alow bulk density. Generally, large-surface oxides of different metalscan be produced by means of this method. Such oxides are advantageouslyproduced from the group of metals consisting of silicon, aluminum,titanium, zirconium, cerium or mixtures of such metals.

So that the powder of the powder-gas aerosol can, for example, depositsufficiently well in the pores of the catalytically coated wall-flowfilter or adhere sufficiently well to the filter wall, the particlediameter in the aerosol should be at least smaller than the pores of thewall-flow filter. This can be expressed by the ratio of the averageparticle diameter (Q₃ distribution, measured according to the mostrecent ISO 13320 on the date of application) d50 in the dry aerosol andthe average pore diameter of the wall-flow filter after coating(measured according to DIN 66134, latest version on the date ofapplication) being between 0.03-2, preferably between 0.05-1.43 and veryparticularly preferably between 0.05-0.63. As a result, the particles ofthe powder in the aerosol, following the gas flow, can precipitate inthe pores of the walls of the wall-flow filter.

The powders should advantageously be fixed to the carrier without prioror subsequent treatment. For a powder suitable for producing the filtersaccording to the invention, an optimization between the largest possiblesurface area of the powder used, the crosslinking and the adhesivestrength is advantageous. During operation in the vehicle, smallparticles follow the flow lines approximately without inertia due totheir low particle relaxation time. A random “trembling movement” issuperimposed on this even, convection-driven movement. Following thistheory, the largest possible flowed-around surfaces should be providedfor a good filtration effect of a filter impinged with powder. Thepowder should therefore have a high proportion of fines, since with thesame total volume of oxide, small particles offer significantly largersurfaces. At the same time, however, the pressure loss must onlyincrease insignificantly. This requires a loose crosslinking of thepowder. For a filtration efficiency-enhancing coating, it is preferableto use powders having a tapped density of between 50 g/l and 900 g/l,preferably between 200 g/l and 850 g/l and very preferably between 400g/l and 800 g/l.

According to the invention, the powders can be used as such as describedabove. However, the use of dry powder which supports a catalyticactivity with regard to exhaust-gas aftertreatment is also conceivable.Accordingly, the powder itself can likewise be catalytically active withregard to reducing harmful substances in the exhaust gas of an internalcombustion engine. Suitable for this purpose are all activities known tothe person skilled in the art, such as TWC, DOC, SCR, LNT orsoot-burn-off-accelerating catalysts. The powder will generally have thesame catalytic activity as an optional catalytic coating of the filter.This further increases the overall catalytic activity of the filter ascompared to filters not coated with catalytically active powder. In thisrespect, it may be possible to use aluminum oxide impregnated with anoble metal for producing the powder-gas aerosol, for example. It isalso possible to apply two or more powders of different composition andfunctionality as dry powder-gas aerosol in two or more successivecoating steps.

In this connection, preference is given to three-way activity of thepowder applied as an aerosol with activation including palladium andrhodium along with an oxygen storage material, such as cerium zirconiumoxide. It is likewise conceivable for catalytically active material tobe used for the SCR reaction. Here, the powder may consist, for example,of zeolites or zeotypes exchanged with transition metal ions. The use ofiron-exchanged and/or copper-exchanged zeolites is very particularlypreferred; extremely preferred as material for producing the powder-gasaerosol is CuCHA (copper-exchanged chabazite;http://europe.iza-structure.org/IZA-SC/framework.php?STC=CHA) or CuAEI(http://europe.iza-structure.org/IZA-SC/framework.php?STC=AEI).

A further advantage is a powder whose catalytic activity leads toimproved soot combustion. A powder consisting of an aluminum oxideimpregnated with one or more noble metals is preferred. Preferred noblemetals in this case are platinum, palladium, rhodium or mixturesthereof. Particularly preferred is an aluminum oxide impregnated withplatinum and palladium. Further materials catalyzing soot burn-off arepure or doped cerium oxides and/or cerium/zirconium mixed oxides. Dopingelements known to the person skilled in the art are those from the groupof rare earth metals, such as lanthanum, yttrium, neodymium,praseodymium. Further known elements catalyzing soot burn-off arederived from the group of alkali metals, alkaline earth metals andtransition metals, such as magnesium, calcium, iron, copper, manganese.Such metals can be applied to the filter either directly in powder form,e.g., as sulfate, carbonate, oxide or analogous compounds, or as acomposite in conjunction with aluminum oxide, cerium oxide and/orcerium/zirconium mixed oxide.

It should be particularly pointed out that the filters described herein,which are impinged with powder, differ from those that are produced inthe exhaust system of a vehicle as a result of ash deposition duringoperation. According to the invention, the filters are deliberatelycoated with various specific, dry powders. As a result, the balancebetween filtration efficiency, catalytic activity and exhaust-gas backpressure can be deliberately adjusted for the intended purpose (dieselsoot, gasoline engine soot) right from the start.

The present invention also provides a method for producing a wall-flowfilter according to the invention. In principle, the person skilled inthe art knows how to produce an aerosol from a powder and a gas in orderto then guide the aerosol through the filter which is to be impinged bythe powder. In order to produce a wall-flow filter for reducing theharmful substances in the exhaust gas of an internal combustion engine,a dry filter provided with a catalytically active coating and havingregions of different permeability is impinged according to the inventionwith powder-gas aerosols on the input side. The filter is impinged withthe powder-gas aerosol by dispersing the powder in a gas, which is thenfed to a gas stream and is subsequently sucked through the filter on theinput side.

The aerosol consisting of the gas and the powder may be produced inaccordance with the requirements of the person skilled in the art or asillustrated below. For this purpose, a powder is usually mixed with agas (http://www.tsi.com/Aerosolgeneratoren-und-dispergierer/;https://www.palas.de/de/product/aerosolgeneratorssolidparticles). Thismixture of gas and powder produced in this way is then advantageouslyfed into the inlet side of the wall-flow filter via a gas stream. Theterm “inlet side” refers to the portion of the filter formed by theinflow channels/input channels. The input surface is formed by the wallsurfaces of the inflow channels/input channels on the input side of thewall-flow filter. The same applies mutatis mutandis to the outlet side.

All gases considered by the person skilled in the art for the presentpurpose can be used as gases for producing the aerosol and for inputtinginto the filter. The use of air is most particularly preferred. However,it is also possible to use other reaction gases which can develop eitheran oxidizing (e.g., O₂, NO₂) or a reducing (e.g., H₂) activity withrespect to the powder used. With certain powders, the use of inert gases(e.g., N₂) or noble gases (e.g., He) may also prove advantageous.Mixtures of the listed gases are also conceivable.

In order to be able to deposit the powder to a sufficient depth into theinlet region E and with good adhesion, a certain suction power isneeded. In orientation experiments for the respective filter and therespective powder, the person skilled in the art can form an idea forhimself in this respect. It has been found that the aerosol (powder/gasmixture) is preferably sucked through the filter at a rate of 5 m/s to60 m/s, more preferably 10 m/s to 50 m/s and very particularlypreferably 15 m/s to 40 m/s. This likewise achieves an advantageousadhesion of the applied powder.

Dispersion of the powder in the gas for establishing a powder-gasaerosol takes place in various ways. Preferably, the dispersion of thepowder is generated by at least one or a combination of the followingmeasures: compressed air, ultrasound, sieving, “in situ milling,”blowers, expansion of gases, fluidized bed. Further dispersion methodsnot mentioned here can likewise be used by the person skilled in theart. In principle, the person skilled in the art is free to select amethod for producing the powder-gas aerosol. As just described, thepowder is first converted into a powder-gas aerosol by dispersion andthen fed into a gas stream.

This mixture of gas and powder thus produced is only subsequentlyintroduced into an existing gas stream, which carries the finelydistributed powder into the inlet side E of the wall-flow filter. Thisprocess is preferably assisted by a suction device, which is positionedin the pipeline downstream of the filter. This is in contrast to thedevice shown in FIG. 3 of U.S. Pat. No. 8,277,880B, with which thepowder-gas aerosol is produced directly in the gas stream. The methodaccording to the invention allows a much more uniform and good mixing ofthe gas stream with the powder-gas aerosol, which ultimately ensures anadvantageous distribution of the powder particles in the filter in theradial and axial directions and thus helps to make uniform and controlthe deposition of the powder particles on the input surface of thefilter. The powder is dry when the wall-flow filter is impinged withinthe meaning of the invention. The powder is preferably mixed withambient air and applied to the filter. By mixing the powder-gas aerosolwith particle-free gas, preferably dry ambient air, the concentration ofthe particles is reduced to such an extent that no appreciableagglomeration takes place until deposition in the wall-flow filter. Thispreserves the particle size in the aerosol adjusted during thedispersion.

A preferred device for producing a wall-flow filter according to theinvention is schematically illustrated in FIG. 12 . Such a device ischaracterized in that it comprises

at least one unit for dispersing powder in a gas;

a unit for mixing the dispersion with an existing gas stream;

at least two filter-receiving units designed to allow the gas stream toflow through the filter without further supply of a gas;

a suction-generating unit that maintains the gas stream through thefilter;

optionally, a unit for generating vortices upstream of the filter, sothat a deposition of powder on the input plugs of the filter isprevented as far as possible; and

optionally, one unit through which at least a partial gas stream isextracted downstream of the suction device and added before the powderaddition to the gas stream that is sucked through the filter.

In a preferred embodiment of the method according to the invention, asshown in the drawing of FIG. 12 , at least a partial gas stream isextracted downstream of the suction device and added before the powderaddition back to the gas stream that is sucked through the filter. Thepowder is thereby metered into an already heated air stream. The suctionblowers for the necessary pressures generate approximately 70° C.exhaust air temperature since the installed suction power ispreferably >20 kW. In an energetically optimized manner, the waste heatof the suction blower is used to heat the supply air in order to reducethe relative humidity of the supply air. This in turn reduces theadhesion of the particles to one another and to the input plugs. Thedeposition process of the powder can thus be better controlled.

In the present method for producing a wall-flow filter, a gas stream isloaded with a powder-gas aerosol and sucked into a filter. This ensuresthat the powder can be distributed sufficiently well in the gas streamfor it to be able to penetrate into the inlet channels of the filter onthe inlet side of the wall-flow filter. Homogeneous distribution of thepowder in the gas/air requires intensive mixing. For this purpose,diffusers, venturi mixers and static mixers are known to the personskilled in the art. Particularly suitable for the powder coating processare mixing devices that avoid powder deposits on the surfaces of thecoating system. Diffusers and venturi tubes are thus preferably used forthis process. The introduction of the dispersed powder into afast-rotating rotating flow with a high turbulence has also proveneffective.

In order to achieve an advantageously uniform distribution of the powderover the cross section of the filter, the gas transporting the powdershould have a piston flow (if possible, the same velocity over the crosssection) when impinging on the filter. This is preferably set by anaccelerated flow upstream of the filter. As is known to the personskilled in the art, a continuous reduction of the cross section withoutabrupt changes causes such an accelerated flow, described by thecontinuity equation. Furthermore, it is also known to the person skilledin the art that the flow profile is thus more closely approximated to apiston profile. For the targeted change of the flow, built-incomponents, such as sieves, rings, disks, etc. below and/or above thefilter can be used.

In a further advantageous embodiment of the present method, theapparatus for powder coating has one or more devices (turbulators,vortex generators) with which the gas stream carrying the powder-gasaerosol can be vortexed prior to impinging on the filter. As an examplein this respect, corresponding sieves or grids can be used, which areplaced at a sufficient distance upstream of the filter. The distanceshould not be too large or small so that sufficient vortexing of the gasstream directly upstream of the filter is achieved. The person skilledin the art can determine the distance in simple experiments. Theadvantage of this measure is explained by the fact that powderconstituents do not deposit on the inlet plugs of the outlet channelsand all the powder can penetrate into the input channels. Accordingly,it is preferred according to the invention if the powder is vortexedbefore flowing into the filter in such a way that deposits of powder onthe inlet plugs of the wall-flow filter are avoided as far as possible.A turbulator or turbulence or vortex generator in aerodynamics refers toequipment which causes an artificial disturbance of the flow. As isknown to the person skilled in the art, vortices (in particularmicrovortices) form behind rods, gratings, and other flow-interferingbuilt-in components at corresponding Re numbers. Known is the Karmanvortex street (H. Benard, C. R. Acad. Sci. Paris. Ser. IV 147, 839(1908); 147, 970 (1908); T. von Karman, Nachr. Ges. Wiss. Göttingen,Math. Phys. KI. 509 (1911); 547 (1912)) and the wake turbulence behindairplanes which can cover roofs. In the case according to the invention,this effect can be intensified very particularly advantageously byvibrating self-cleaning sieves (so-called ultrasonic screens) whichadvantageously move in the flow. Another method is the disturbance ofthe flow through sound fields, which excites the flow to turbulences asa result of the pressure amplitudes. These sound fields can even cleanthe surface of the filter without flow. The frequencies may range fromultrasound to infrasound. The latter measures are also used for pipecleaning in large-scale technical plants.

The preferred embodiments for the wall-flow filter apply mutatismutandis also to the method. Reference is explicitly made in thisrespect to what was said above about the wall-flow filter.

The present invention also relates to the use of a wall-flow filteraccording to the invention for reducing harmful exhaust gases of aninternal combustion engine. In principle, all catalytic exhaust-gasaftertreatments (see above) coming into consideration for this purposeto the person skilled in the art and having a filter can serve forapplication purposes, but in particular those with which the filter isin an exhaust system together with one or more catalytically activeaggregates selected from the group consisting of nitrogen oxide storagecatalysts, SCR catalysts, three-way catalysts and diesel oxidationcatalysts. The filter according to the invention is particularlyadvantageously used in combination with a three-way catalyst, inparticular on its downstream side. It is particularly advantageous ifthe filter itself is a three-way catalytically active filter. Thefilters produced by the method according to the invention, optionallycoated with catalytically active powder, are suitable for all theseapplications. The use of the filters according to the invention for thetreatment of exhaust gases of a stoichiometrically operated internalcombustion engine is preferred. The preferred embodiments described forthe wall-flow filter according to the invention also apply mutatismutandis to the use mentioned here.

The requirements applicable to gasoline particulate filters differsignificantly from the requirements applicable to diesel particulatefilters (DPF). Diesel engines without DPF can have up to ten timeshigher particle emissions, based on the particle mass, than gasolineengines without GPF (Maricq et al., SAE 1999-01-01530). In addition,there are significantly fewer primary particles in the case of gasoline,engines and the secondary particles (agglomerates) are significantlysmaller than in diesel engines. Emissions from gasoline engines rangefrom particle sizes of less than 200 nm (Hall et al., SAE 1999-01-3530)to 400 nm (Mathis et al., Atmospheric Environment 38 4347) with amaximum in the range of around 60 nm to 80 nm. For this reason, thenanoparticles in the case of GPF must mainly be filtered by diffusionseparation. For particles smaller than 300 nm, separation by diffusion(Brownian molecular motion) and electrostatic forces becomes more andmore important with decreasing size (Hinds, W.: Aerosol technology:Properties and behavior and measurement of airborne particles. Wiley,2nd edition 1999).

Dry in the sense of the present invention accordingly means exclusion ofthe application of a liquid, in particular water. In particular, theproduction of a suspension of the powder in a liquid for spraying into agas stream should be avoided. A certain moisture content may possibly betolerable both for the filter and for the powder, provided thatachieving the objective, the most finely distributed deposition of thepowder possible in or on the input surface, is not negatively affected.As a rule, the powder is free-flowing and dispersible by energy input.The moisture content of the powder or of the filter at the time ofapplication of the powder should be less than 20%, preferably less than10% and very particularly preferably less than 5% (measured at 20° C.and normal pressure, ISO 11465, latest version on the date ofapplication).

The wall-flow filter produced according to the invention andcatalytically coated exhibits an excellent filtration efficiency withonly a moderate increase in exhaust-gas back pressure as compared to awall-flow filter in the fresh state that has not been impinged bypowder. The wall-flow filter according to the invention preferablyexhibits an improvement in soot particle deposition (filtering effect)in the filter of at least 5%, preferably at least 10% and veryparticularly preferably at least 20% at a relative increase in theexhaust-gas back pressure of the fresh wall-flow filter of at most 40%,preferably at most 20% and very particularly preferably at most 10% ascompared to a fresh filter coated with catalytically active material butnot treated with powder. As stated, in a very preferable embodiment, thepowder deposits only into the open pores of the filter and forms aporous matrix there. The slight increase in back pressure is probablydue to the cross section of the channels on the input side not beingsignificantly reduced by impinging, according to the invention, thefilter with a powder. It is assumed that the powder in itself forms aporous structure, which is believed to have a positive effect on theback pressure. For this reason, a filter according to the inventionshould also exhibit better exhaust-gas back pressure than those of theprior art, with which a powder was deposited on the walls of the inletside of a filter or a traditional coating using wet techniques waschosen.

Since the wall-flow filter, which contains one or more zoned catalyticcoatings on the surfaces of the channel walls, is provided for reducingthe harmful substances in the exhaust gas of an internal combustionengine, wherein the dry filter was deliberately impinged on its inputsurface with a dry powder-gas aerosol, which advantageously respectivelyhas at least one compound with a high melting point, an extremelysuccessful solution to the object posed is achieved. The invention isexplained in more detail with reference to some examples and figures.

FIG. 1 shows a light microscopy image of a filter impinged with powder.The photograph shows a top view of a plurality of channel walls in aregion of the filter where no catalytically active on-wall layer islocated. The powder is selectively deposited in the pores of the walland fills them.

FIGS. 2-8 show different coating arrangements in catalytically activeparticulate filters according to the invention. The followingdesignations are used therein:

(E) the input channel/inflow channel of the filter(A) the output channel/outflow channel of the filter(L) the length of the filter wall(X) catalytic in-wall coating(Y) and (Z) catalytic on-wall coating(P) regions of the filter wall impinged with powder

FIG. 2 schematically shows a filter which has on the inlet side (Y) andoutlet side (Z) one zone each of an on-wall layer with a zone length ofapprox. 60% of the total length and is impinged with powder (P) at theend in the uncoated part of the inlet channel (E).

FIG. 3 shows the same structure of the filter as FIG. 2 , with thedifference that the two on-wall layers (Y, Z) have a coating gradientfrom the end faces toward the center of the carrier.

FIG. 4 shows a filter with an on-wall layer (Y) in the inlet region E,which is impinged with a powder (P) in the inlet channel (E).

FIG. 5 shows a filter with an on-wall layer (Z) in the outlet region Aand a zone with an in-wall coating (X), with which the powder (P)precipitates in the region of highest permeability.

FIG. 6 shows a filter with which the walls of the channels arecompletely activated with an in-wall coating (X) and which on the inletside (E), has a zone with an on-wall layer (Y) and was impinged withpowder (P).

FIG. 7 shows the same filter as FIG. 5 , but without catalyticallyactive in-wall coating (X).

FIG. 8 shows schematically a filter with an on-wall layer each in theinput channel (Y) and output channel (Z) with a length of approximately30% of the total length L and a region in the input channel (E) impingedwith powder (P).

FIG. 9 shows the results of the investigations on the filtration effectof all described particulate filters according to the invention andparticulate filters according to the prior art.

FIG. 10 shows a photograph of an opened filter with a zone of an on-walllayer in the inlet channels (gray, left side) and the region impingedwith powder (right side).

FIG. 11 shows enlargements of the filter of FIG. 10 with photos of theinput (1), the center (2) and the output (3) of the filter.

FIG. 12 shows a schematic drawing of an advantageous device forimpinging the filters with a powder. Together with the gas, the powder420 or 421 is mixed with the gas stream 454 under pressure 451 by theatomizer nozzle 440 in the mixing chamber and then sucked or pushedthrough the filter 430. The particles that passed through are filteredout in the exhaust gas filter 400. The blower 410 provides the necessaryvolumetric flow. The exhaust gas is divided into an exhaust gas 452 anda warm cycle gas 453. The warm cycle gas 453 is mixed with the fresh gas450.

EXAMPLES Producing and Testing the Particulate Filters

Conventional high-porosity cordierite filters having a round crosssection were used to produce the catalytically active particulatefilters described in examples and comparative examples. The wall-flowfilter substrates had a cell density of 46.5 cells per square centimeterat a cell wall thickness of 0.22 mm. They had a porosity of 65% and anaverage pore size of 18 μm.

In Comparative Example 1, a coating suspension was applied in theoutflow channels. In Comparative Example 2, coating suspensions wereapplied in two steps in the inflow channels and outflow channels. Thefilter described in Comparative Example 3 contains both a coatingarranged in the wall and a coating suspension applied in the inflowchannels. After the application of each coating suspension, thewall-flow filters were dried and calcined at 500° C. for the duration of4 hours. The coating suspension was applied according to therequirements of the person skilled in the art (as described inDE102010007499A1).

In the case of the particulate filters according to the invention(Examples 1 to 5), the filters from Comparative Examples 1-3 wereadditionally impinged with a powder in the input channels.

The catalytically active filters thus obtained were investigated fortheir fresh filtration efficiency on the engine test bench in the realexhaust gas of a motor operated predominantly (>50% of the operatingtime) and on average (mean lambda over the running time) with astoichiometric air/fuel mixture. A globally standardized test procedurefor determining exhaust emissions, or WLTP (Worldwide harmonized Lightvehicles Test Procedure) for short, was used here. The driving cycleused was WLTC Class 3. The respective filter was installed close to theengine immediately downstream of a conventional three-way catalyst. Thisthree-way catalyst was the same for all filters measured. Each filterwas subjected to a WLTP. In order to be able to detect particulateemissions during testing, the particle counters were installed upstreamof the three-way catalyst and downstream of the particulate filter.

Some catalytically active particulate filters were also additionallysubjected to engine test bench aging. The aging process consisted of anoverrun cut-off aging process with an exhaust gas temperature of 950° C.before the catalyst input (maximum bed temperature of 1030° C.). Theaging time was 38 hours. After aging, the filters were investigated fortheir catalytic activity.

In the analysis of catalytic activity, the light-off behavior of theparticulate filters was determined at a constant average air ratio A onan engine test bench, and the dynamic conversion was checked when Achanged.

Comparative Example 1

On wall-flow filters with a diameter of 118 mm and a length of 152 mm, anoble metal-containing coating suspension containing a cerium/zirconiummixed oxide, a lanthanum-doped aluminum oxide and barium sulfate wasapplied to 80% of the length of the output channel of the filter andsubsequently calcined at 500° C. The grain size of the oxides of thecoating suspension was selected such that the suspension was appliedpredominantly to the filter wall (only a small amount of the fraction ofultra-fine coating particles penetrates into the pores of the wall; lessthan 10%). After calcination, the coating amount of the VGPF1corresponded to 67 g/l based on the volume of the substrate.

Comparative Example 2

On wall-flow filters with a diameter of 118 mm and a length of 118 mm, anoble metal-containing coating suspension containing a cerium/zirconiummixed oxide, a lanthanum-doped aluminum oxide and barium sulfate wasapplied in a first step to 60% of the length of the input channel of thefilter and subsequently calcined. In a second coating step, a furthernoble metal-containing coating suspension was applied to 60% of thelength of the output channel and subsequently calcined. The coatingsuspension used in the second coating step also contained acerium/zirconium mixed oxide, a lanthanum-doped aluminum oxide andbarium sulfate. The grain size of the oxides of the coating suspensionwas selected such that the suspension was applied predominantly to thefilter wall. The amount of coating of the VGPF2 after calcinationcorresponded to 100 g/l based on the volume of the substrate.

Comparative Example 3

On a highly porous wall-flow filter with a diameter of 132 mm and alength of 102 mm, a noble metal-containing coating suspension containinga cerium/zirconium mixed oxide, a lanthanum-doped aluminum oxide andbarium sulfate was applied in a first step to the entire length of thefilter and subsequently calcined. The grain size of the oxides of thecoating suspension was selected such that the suspension ispredominantly located in the filter wall (>90%). The amount of coatingafter calcination corresponded to 100 g/l based on the volume of thesubstrate.

In a second coating step, a further noble metal-containing coatingsuspension was applied to 60% of the input channel and subsequentlycalcined. The coating suspension used in the second coating step alsocontained a cerium/zirconium mixed oxide, a lanthanum-doped aluminumoxide and barium sulfate. The grain size of the oxides of the coatingsuspension was selected such that the suspension was appliedpredominantly to the filter wall. The amount of coating of the VGPF3after calcination corresponded to 132 g/l based on the volume of thesubstrate.

In order to increase the filtration efficiency of the catalyticallycoated filters described in Comparative Examples 1 to 3, the inflowchannels thereof were impinged with various amounts and types of powder.In this case, the coating parameters were chosen such that the powderused was deposited mainly in the region of the substrate in which therewas no on-wall coating (points of highest permeability). The productionof the filters GPF1 to GPF5 according to the invention is explained inthe following descriptions.

In the examples, the influence of the type and amount of powder used indifferent coating variants of the catalytic materials on filtrationefficiency and catalytic activity was investigated.

Example 1

In order to increase the filtration efficiency of the catalyticallycoated filter VGPF1 described in Comparative Example 1, the inflowchannels of the filter were impinged with 4 g/l of a highly porousaluminum oxide. The relative back pressure increase of the GPF1 comparedto the VGPF1 was 8.6 mbar. The filter GPF1 described in Example 1 isoutlined in FIG. 7 .

Example 2

In order to increase the filtration efficiency of the catalyticallycoated filter VGPF1 described in Comparative Example 1, the inflowchannels of the filter were impinged with 0.2 g/l of a pyrogenicaluminum oxide with a high melting point. The relative back pressureincrease of the GPF1 compared to the VGPF1 was 5 mbar. The filter GPF2described in Example 2 is outlined in FIG. 7 .

Example 3

In order to increase the filtration efficiency of the catalyticallycoated filter VGPF2 described in Comparative Example 2, the inflowchannels of the filter were impinged with 10 g/l of a highly porousaluminum oxide. The filter GPF3 described in Example 3 is outlined inFIG. 2 .

Example 4

In order to increase the filtration efficiency of the catalyticallycoated filter VGPF3 described in Comparative Example 3, the inflowchannels of the filter were impinged with 4 g/l of a highly porousaluminum oxide. The filter GPF4 described in Example 4 is outlined inFIG. 4 .

Example 5

In order to increase the filtration efficiency of the catalyticallycoated filter VGPF3 described in Comparative Example 3, the inflowchannels of the filter were impinged with 7 g/l of a highly porousaluminum oxide. The filter GPF5 described in Example 5 is outlined inFIG. 4 .

Discussion of the Results from Filtration Efficiency Measurements of theParticulate Filters VGPF1 to VGPF3 Along with GPF1 to GPF5 Described inComparative Examples and Examples

As already described, the catalytically active filters produced incomparative examples and examples were each subjected to a WLTP on anengine test bench in order to investigate their filtering effect. Theresults from these investigations were shown in FIG. 9 . FIG. 9 showsfiltration values that have resulted from the raw particle emissions andparticle emissions after the respective filter during a WLTP procedure.

The advantages of the filters GPF1 to GPF5 according to the inventioncan be clearly observed during the filtering effect measurement.Impinging the filter with a powder results in a filtration efficiencyincrease of up to 20%. The desired filtration efficiency can be adjustedby the quantity of powder used.

Replacing a highly porous aluminum oxide (GPF1) with a pyrogenicaluminum oxide (GPF2) reduces the amount of powder used from 4 g/l to0.2 g/l. This leads to a saving of the powder used by 500% by weightwith an unchanged filtration effect and a lower back pressure.

In order to check whether the filters impinged with powder have highcatalytic activity, the particulate filters GPF4 and GPF5 were subjectedto engine test bench aging and a subsequent measurement of the light-offbehavior.

The table below contains the temperatures T₅₀ at which 50% of theconsidered components are respectively converted. In this case, thelight-off behavior with stoichiometric exhaust gas composition (λ=0.999with ±3.4% amplitude) was determined. The standard deviation in thistest is ±2° C.

Table 1 contains the light-off data for the aged filters VGPF3, GPF4 andGPF5.

TABLE 1 T₅₀ HC T₅₀ CO T₅₀ NOx stoichiometric stoichiometricstoichiometric VGPF3 381 398 395 GPF4 386 401 401 GPF5 383 401 402

As the results show, impinging the catalytically active filters with apowder leads to a significant increase in filtration efficiency with anunchanged high catalytic activity and a low back pressure rise. Thechoice of powder may also significantly reduce the amounts of powderused.

It has been shown successfully that the catalytic activity of thezone-coated wall-flow filters, the exhaust-gas back pressure and thefiltration efficiency can be adapted to the customer requirements in atargeted manner. A correspondingly produced wall-flow filter was not yetknown from the prior art.

1. A wall-flow filter for removing particles from the exhaust gas ofinternal combustion engines, which comprises a wall-flow filter oflength L and at least one catalytically active coating Y and/or Z,wherein the wall-flow filter comprises channels E and A, which extend inparallel between a first and a second end of the wall-flow filter andare separated by porous walls, which form surfaces O_(E) and O_(A),respectively, and wherein the channels E are closed at the second endand the channels A are closed at the first end, the coating Y is locatedin the channels E on the walls of the surfaces O_(E), wherein thecoating on the walls of the surface O_(E) extends from the first end ofthe wall-flow filter to a length of less than the length L, the coatingZ is located in the channels A on the walls of the surfaces O_(A),wherein the coating on the walls of the surface O_(A) extends from thesecond end of the wall-flow filter to a length of less than the lengthL, wherein the inlet region E of the filter has additionally beenimpinged with at least one dry powder-gas aerosol where the dry powder Pis a high-melting metal compound where such corresponding metalscompounds are metal oxide, metal sulfate, metal phosphate, metalcarbonate or metal hydroxide powders or their mixtures and are selectedof the group of calcium, magnesium, strontium, barium, aluminum,silicon, titanium, zirconium, cerium, and wherein the dry powder P fillsup one or both of (i) and (ii), with (i) being cracks or interruptionsformed in the dried, or dried and sintered, catalytically active coatingY, when present, and (ii) being pores exposed in the surface O_(E).2.-17. (canceled)
 18. The wall-flow filter of claim 1, wherein coating Yis present.
 19. The wall-flow filter of claim 18, wherein coating Yextends for less than the length L and a full length of the remaining,exposed region of surface O_(E) has pores filled with the dry powder P.20. The wall-flow filter of claim 18, wherein coating Z is present. 21.The wall-flow filter of claim 20, further comprising a catalytic coatingX that is located in the walls of the wall-flow filter.
 22. Thewall-flow filter of claim 21, wherein a full length of coating X alsohas dry powder P present for at least that length as well.
 23. Thewall-flow filter of claim 1, wherein coating Y is not present andcoating Z is present and the dry powder sufficiently coincides relativeto length L with coating Z such that exhaust flow through dry-powder Ppasses through a region of coating Z before exiting.
 24. The wall-flowfilter of claim 23, further comprising a catalytic coating X that islocated in the walls of the wall-flow filter and a combined length ofcoverage of dry powder P and coating Z is at least 100% of L.
 25. Thewall-flow filter of claim 1, wherein dry powder P has a heaviestpresence closer to the first end than the second end.
 26. The wall-flowfilter of claim 1, wherein the dry powder P fills the pores in theexposed surface O_(E) only as far as an input surface of the exposedsurface O_(E) as not to form a cohesive layer on the exposed surfaceO_(E).
 27. The wall-flow filter of claim 26, wherein the total amount ofdry powder retained on the wall-flow filter is not greater than 5 g/l.28. The wall-flow filter of claim 1, wherein Y is present for a lengthof 60% to 80% of L with the remaining 40% to 20% of channel E havingpores filled with dry powder P.
 29. The wall-flow filter of claim 1,wherein each of Y and Z are present for less than 100% of L as to leavean intermediate region of the wall-flow filter free of Y and Z coating,and which intermediate region is where dry-powder P has a heaviestpresence.
 30. The wall-flow filter of claim 1, wherein a ratio of theaverage particle diameter d50 of dry-powder P and average pore diameterof the pores exposed in the surface O_(E) is from 0.05 to 1.43.
 31. Thewall-flow filter of claim 1, wherein a ratio of the average particlediameter d50 of dry-powder P and average pore diameter of the poresexposed in the surface O_(E) is from 0.05 to 0.63.
 32. The wall-flowfilter of claim 1, wherein the dry-powder supports catalytically activematerial.
 33. The wall-flow filter of claim 32, wherein thecatalytically active material includes noble metals.
 34. The wall-flowfilter of claim 1, wherein the tapped density of the dry powder is atmost 900 kg/m².
 35. A method for producing a catalytically activewall-flow filter according to claim 1, wherein the catalytically coatedfilter is impinged with the powder-gas aerosol by dispersing the powderin a gas, which is then fed to a gas stream and is subsequently suckedthrough the filter on the input side.
 36. The method for producing acatalytically active wall-flow filter according to claim 35, wherein theaerosol is sucked through the filter at a rate of 5 m/s to 50 m/s andthe dry powder has a moisture content of less than 20% at the time ofimpingement on the wall-flow filter.
 37. A method of reducing harmfulexhaust gases comprising passing the harmful exhaust cases through thecatalytically active wall-flow filter according to claim 1.